1. Field of the Invention
The present invention is related to a tool for machining an initial internal configuration in the solid propellant grain of a rocket motor. More particularly, the present invention is related to a device and method for machining radial slots in propellant grain by cutting a portion of propellant grain away along a path and removing the portion of propellant grain in a plurality of pieces.
2. Technical Background
When a solid rocket motor is ignited, the ignition spreads to the entire exposed surface of the inner bore of the motor within milliseconds. The motor burns from the inner bore to the case wall until all of the propellant has been ignited.
The amount of propulsion provided by the rocket motor at a given time during a burn can be generally determined by knowing the area of the surface being burned. Thus, the firing characteristics of a solid rocket motor are largely determined by the initial internal configuration of the inner bore of the motor.
For example, in a solid rocket motor having a generally cylindrical inner bore, the surface area of the inner bore will increase over the time of the burn because the diameter of the inner bore increases as the propellant is burned. Thus, the amount of propulsion provided by such a motor will generally increase over the time of the burn.
As the propellant of a solid rocket motor burns, the mass of propellant is progressively depleted. The rate at which the propellant is consumed largely determines the level of thrust generated by the motor. It has been determined that by altering the initial internal configuration of the propellant with radial, circumferential slots, some control overt the rate the propellant is consumed during the burn can be maintained.
One method of producing a rocket motor having an internal configuration with radial, conical slots is to mold the slots into the motor. In the manufacture of solid rocket motors, propellant, in liquid form, is poured into the vertically oriented rocket casing which has been fit with a casting core. The casting core is a displacement type device used to produce the desired internal configuration of the propellant.
After the propellant has cured, the core is extracted from the motor through the aft opening in the case. In instances where circumferential radial slots are cast into the propellant, the portion of the core which forms the slots must be detachable from the central portion of the core. The portion of the core forming the slot is typically made of a foam filler, which can be left in the motor for firing, or silicone rubber which must be manually removed from the motor prior to filing.
If the silicone rubber molds are utilized, extreme caution must be employed when removing the radial slot moldings to ensure that the propellant is not pinched. Because of the relative movement of the moldings and the propellant, the danger of an electrostatic buildup exists. Electrostatic discharge in the form of a spark in the presence of propellant under pressure could result in ignition of the propellant. The mere possibility of inadvertent ignition is a principal disadvantage to the use of molds to configure circumferential slots in a solid rocket motor. Additionally, high labor costs and time requirements associated with the molding of these slots render their use disadvantageous.
Another disadvantage to the use of molds to form circumferential slots in the propellant is the lack of flexibility to tailor the grain design to satisfy unique requirements of various users of solid rocket motors. Each design requires that a separate casting core be produced. Because of the complexity of the casting core, they tend to be quite expensive. Additionally, the lead time required for producing such a casting core can be as great as one year.
An additional disadvantage to molding circumferential slots in the rocket motor is the geometrical limitations on the sizes and orientation of the slots which may be molded into the propellant. Circumferential slots molded into the propellant generally must be quite shallow. If a deep slot is molded into the propellant, the mold can be virtually impossible to safely remove. Additionally, the presence of a mold extending close to the casing wall can substantially interfere with pouring the propellant into the casing.
An alternate way of configuring the interior of a solid rocket motor with circumferential radial slots is to machine the slots into the propellant by utilizing a rotatably mounted cutter. By adjusting the radial and axial location of the cutter with each rotation, the cutter slowly forms the radial, circumferential slot within the propellant by successively paring out ribbons of propellant.
A significant disadvantage to this machining process is that electrostatic buildup occurs within the motor from the relative movement of propellant ribbons across adjacent propellant surfaces. Because of the sensitive nature of the propellant grain, any significant buildup of electrostatic charge should be avoided.
Another problem associated with the use of the machining process described above is that the ribbon of propellant produced by the cutter is particularly difficult to remove. Removal of the propellant ribbon is accomplished by frequently removing the tool from the radial slot and vacuuming the ribbon from the slot and from the interior of the rocket motor. Additionally, ribbons of propellant do not lend themselves to easy removal with a vacuum hose, as they tend to adhere to the hose interior thereby plugging the vacuum system. Also, ribbons of cut propellant left within the slot interfere with the proper operation of the cutter. Thus, cutting a circumferential slot into the grain is extremely time consuming because the tool must be repeatedly removed from the motor to clear the cut propellant out of the motor.
Thus, it would be an advancement in the art to provide an apparatus and method for forming circumferential, radial slots in the propellant grain of a solid rocket motor which minimizes electrostatic buildup, thereby reducing the danger generally associated forming circumferential radial slots in propellant grain.
It would also be an advancement in the art to provide an apparatus and method for forming circumferential, radial slots in the propellant grain of a solid rocket motor which provides a method for continuously removing the propellant cut away from the rocket motor during the process.
It would be a further advancement in the art to provide such an apparatus and method which is versatile in that it could be used to tailor the grain of a general design to satisfy specific manufacturing requirements while eliminating the extreme lead time and expense of preparing new casting tooling for each design.
It would be yet another advancement in the art to provide such methods and apparatus which could machine circumferential slots in a solid rocket motor and effectively and efficiently expel the propellant removed from the rocket motor in forming the slots.
It would be an additional advancement in the art if such methods and apparatus could provide a cost-efficient means for configuring an initial internal configuration in a solid rocket motor.
Such a device and method are disclosed and claimed herein.